Pure cultures will be studied singly and in combination to elucidate how organisms interact to produce characteristic fermentations of complex, anaerobic, microbial ecosystems. The studies will focus on mammalian gastro-intestinal fermentations and anaerobic waste decomposition. Previous investigations showed that H2-using species can increase formation of H2 and alter fermentation products of major saccharolytic rumen species. These studies will be extended to other H2-producing saccharolytic species of the rumen, the human large intestine and to organisms that produce H2 from non-carbohydrate sources. Interactions between cellulose- or starch-hydrolyzing species and non-polymer fermenting species will be examined. The latter organisms use soluble sugar intermediates produced from the polymers. A catalogue of species capable of interaction will be prepared and the amount of competition for hexose will be estimated. Models of the various ecosystem fermentations will be prepared by mixing selected pure cultures. To model the systems that completely convert organic carbon to CH4 and CO2, the nutrition of the only pure culture known to convert acetate to CH4 and CO2 will be studied, and isolation of new species will be undertaken. Changes in growth rate, controlled by limiting carbohydrate in a chemostat, markedly change the fermentation products of certain bacteria by affecting paths of pyruvate catabolism. Studies will be extended to other species with alternate paths of pyruvate catabolism to examine the generality of the phenomenon. The questions of whether limiting nutrients other than carbohydrates will produce the same effects and whether control of pathways is pre- or post-translational will be examined. Growth rate-fermentation product relationships may be an important feature of ecosystem activity. Studies will be continued of electron transport systems that are significant in the production of H2 by organisms whose fermentations are influenced by H2. BIBLIOGRAPHIC REFERENCES: Wolin, M.J. 1975. Interactions between the bacterial species of the ruman, p. 134-148. In: I.W. McDonald and A.C.I. Warner (Eds.), Digestion and metabolism in the ruminant. The University of New England Publishing Unit, Armidale, Australia.